The present invention relates to toasting devices for toasting food which automatically control the degree of browning of food toasted within them. The present invention particularly relates to such toasters that use a sensor to sense some quality of the food being toasted.
Known techniques for controlling the level of browning of bread being toasted in a toaster include
the use of thermostatic devices such as bi-metallic strip devices;
the use of timing devices such as electronic timers;
the use of temperature sensors to monitor the temperature of the food being toasted (for example, see GB-A-2 193 086);
the use of optical devices which sense the colour of food as it is toasting, in conjunction with an electronic controller for stopping the toasting process (for example, see EP-A-0 033 642); and
the use of electro-chemical sensors for sensing the quantities and qualities of certain gases being given off by the toasting food, in conjunction with an electronic controller for stopping the toasting process (for example, see DE-A-3 842 303).
A problem with the use of thermostatic, timing devices and temperature sensors is that these require an operator to guess at the time needed to reach an ideal browning of the food. If a variety of different foodstuffs are to be inserted into the toaster, each of which has different qualities such as moisture content, density and/or temperatures, the operator is unlikely to be able to set the timer to the correct setting except by experimenting and remembering the correct timing required for each particular foodstuff. An incorrectly set thermostat or timer results in under-toasted bread or burnt toast.
In order to toast frozen foodstuffs, some electronic toasters have a additional xe2x80x98defrostxe2x80x99 mode which extends the time of heating. One disadvantage of such prior art toasters is that a user must remember to select the defrost mode. Another disadvantage is that a user can still not be sure whether a frozen foodstuff will be over under-toasted.
A problem with the use of optical devices in sensing the colour of food being toasted is that it is a complex process and toasters which employ this technique suffer from the problems of being unreliable and overly expensive. It is also not always clear which particular part of a foodstuff, which is being toasted, needs to be xe2x80x98looked atxe2x80x99 by such optical devices.
A problem with sensing the gases coming off of food being toasted is that it involves the use of sensitive, specialised sensors.
Prior art toasters often suffer from problems that elements of the prior art toasters store heat after an initial toasting cycle and due to the manner in which prior art toasters are controlled this stored heat can lead to over-toasting of food in an immediately subsequent toasting cycle if a user does not change various settings on the toaster.
It would be desirable for a toaster to toast food correctly, independent of the toasting temperature and the type and temperature of food being toasted whilst using robust, readily available sensors.
Also known in the art are toasters, which incorporate an optical safety device, which sense smoke given off by food burning in the toaster and cutt-off the power supply to the toaster elements to prevent fire.
It is an object of the present invention to provide a toaster, which toasts food correctly, independent of the toasting temperature and the type and temperature of food being toasted.
It is further object of the present invention to provide a sensing means for such a toaster, which senses the degree of browning of food being toasted by an alternative method to those used in the prior art.
The present invention provides a toasting device comprising a particulate sensor for sensing particulates in the air in the vicinity of food being toasted in the toasting device and an electronic browning control means which controls the extent of browning of the food being toasted by controlling the length of time of toasting based on the output signal of the particulate sensor.
In one preferred embodiment, the particulate sensor comprises an ionisation chamber, an electrical-current sensing means and an electrical power source in an electrical sensing circuit, whereby particulates in the air in the vicinity of food being toasted are sensed due to such particulates causing a change in the current flowing through the electrical sensing circuit.
A toasting device according to the present invention can be used to toast a variety of food with different characteristics e.g. thin dry bread or frozen thick bread or other food stuffs (e.g. bagel, bun etc.) without a user having to alter any settings and to achieve a consistent level of browning for each one. Therefore no pre-guessing is required to achieve the desired level of toasting. The toasting device can be safely left to toast food without any worry of the food either under-toasting or burning.
The particulate sensor effectively senses qualities of the food being toasted and not the temperature of the toasting device itself. This means that a toasting device according to the invention is unaffected by elements of the toasting device storing heat after an initial toasting cycle.
The fact that a particulate sensor is being used for browning control in a toasting device according to the present invention has the additional benefit that no further fire-hazard fail-safe devices are required on the toasting device (such as an optical safety device as mentioned above).